1. Field of the Invention
This invention relates to an improvement in a motor speed control apparatus for the electrically-driven sewing machine capable of effecting single stitch.
2. Description of the Prior Art
The conventional speed control apparatus for the electrically-driven sewing machine capable of effecting single stitch comprises a main motor circuit for supplying a source voltage to the motor through a semiconductor element with a controlled electrode, a first control means for controlling the semiconductor element in order to effect normal operation, a controller adapted to change between its first and second state so as to control the speed of the motor continuously when the first control means is energized, a second control means for controlling the semiconductor element in order to effect single stitch, and change-over means for changing over the operation between the first and second means. The second control means includes a needle position detector means for detecting the position of a needle bar, a switch means connected in series with the needle position detector means, a trigger voltage supply means for applying a trigger voltage to the gate electrode of the semiconductor element, and a switch-operating means having a charge-discharge circuit for closing the switch means when the controller is set in the first state, and for opening the switch means for a predetermined period of time after the time point at which the controller is set in the states between the second state and the predetermined state between the first and second states. When the needle position detector means detects that the needle has reached the predetermined position with the switch means closed, the trigger voltage supply means are made ineffective. Such an apparatus is disclosed in the copending U.S. Patent Application entitled as "Electrically driven sewing machine control apparatus", filed Sept. 3, 1976, Ser. No. 720,380, now U.S. Pat. No. 4,078,507 by the same applicants. In this speed control apparatus, it is necessary to supply a large current to the relay coil of the switch-operating means in order to open the reed relay switch. Further, in order to shorten the time required for preparing following single-stitch after the completion of a single-stitch, the capacity of the capacitor included in the charge-discharge circuit is required to be small in order to shorten the discharge time. In this case, however, the charge time of the capacitor is also small, so that the reed relay switch is kept open for a short time. As a result, the trigger voltage supply means cannot be kept effective for a sufficient length of time in the case of requiring a large torque, thereby leading to the likelihood of failure in single-stitch operation. Furthermore, the high cost and bulikiness of the reed switch results in a high product cost and bulkiness of the speed control apparatus.